TAMR is expected to be one of the future generation of magnetic recording technologies that will enable recording at ˜1-10 Tb/in2 data densities. TAMR involves raising the temperature of a small region of a magnetic medium to near its Curie temperature where both of its coercivity and anisotropy are significantly reduced and magnetic writing becomes easier to achieve even with weak write fields characteristic of small write heads in high recording density schemes. In TAMR, optical power from a light source is converted into localized heating in a recording medium during a write process to temporarily reduce the field needed to switch the magnetizations of the medium grains. Thus, with a sharp temperature gradient of TAMR acting alone or in combination with a high magnetic field gradient, data storage density can be further improved with respect to current state of the art recording technology.
In addition to the components of conventional write heads, a TAMR head also typically includes an optical wave guide (WG) and a plasmon antenna or plasmon generator (PG). The waveguide may be directly attached to a laser diode (LD) light source, or in the case of a free space light beam, a lens is used to focus light onto a waveguide inlet. The waveguide serves as an intermediate path to guide light from the LD or free space light beam to the PA or PG where the light optical mode couples to the local plasmon mode of the PA or to the propagating plasmon mode of the PG. After the optical energy is transformed to plasmon energy, either with local plasmon excitation in the PA or with energy transmission along the PG, it is concentrated at the medium location where heating is desired. Preferably, the heating spot is aligned with the magnetic field from the write head to realize optimum TAMR performance.
A thermally assisted magnetic head structure disclosed in U.S. Patent Application Publications 2008/0192376 and 2008/0198496 employs an edge plasmon mode that is coupled to a waveguide as represented in FIG. 1a. Conventional components of a magnetic recording structure are shown as a main pole 1, return pole 3, and write coils 5 formed along an air bearing surface (ABS) 8-8. The wave guide 4 guides the input optical light wave 6 toward the ABS 8-8 in the center cross-sectional view. As shown in the prospective view, plasmon generator 2 has a triangular shape and may extend a certain distance from the ABS before meeting WG 4. Optical wave 6 couples to the edge plasmon (EP) mode 7 that is excited and propagates along the sharp edge 9 of plasmon generator 2 adjacent to the WG 4. Plasmon mode 7 further delivers the optical power toward the ABS and locally heats a medium (not shown) placed underneath the plasmon generator 2. A plasmon generator is typically made of noble metals such as Ag and Au that are known to be excellent generators of optically driven surface plasmon modes. The local confinement of the edge plasmon mode 7 is determined by the angle and radius of the triangle corner. In FIG. 1b, the magnetic field profile and heating profile are shown with slopes 10 and 13, respectively, that have a slight overlap along the dashed vertical line 14-14.
In TAMR recording, it is critical to know the actual light intensity available in the nearfield at the ABS and medium because this detail determines the quality of the data written into the media. Light intensity and therefore energy density in the media can vary as light source (LD) power, head media spacing, media velocity, and device performance change. In order to monitor these changing conditions, an integrated light intensity measurement device is needed. Intensity measurements should be very efficient in the use of light energy so that a higher power light source is not required. It is also desirable to be able to obtain light intensity measurements over existing data tracks in the absence of a magnetic field to determine if the slider is in a loaded or unloaded condition. One method that is currently used to determine the approximate intensity of a light reaching the ABS is to position a photo detector behind the light source and measure the light intensity propogated in a direction away from the ABS since this value is typically about 10% of the intensity directed towards the media. However, this is a relatively inaccurate measurement since the percentage of light propogated from the light source away from the ABS may vary over time. Furthermore, other factors influence the amount of light propogated through the waveguide toward the media such that the light intensity emanating from the light source near the back end of the waveguide does not necessarily correlate well to the light intensity proximate to the ABS. Therefore, a more accurate measurement technique is required for adequate light control in TAMR designs.
A routine search of the prior art resulted in the following references.
In U.S. Pat. No. 6,091,485, a detector is used to determine physical properties of a material by obtaining measurements of reflected light from a layer of the material.
U.S. Patent Application Publication 2008/0204916 discloses how light propagating through a main waveguide is partially diverted into a second waveguide before reaching the ABS, and the second waveguide directs the diverted light to a photo detector for a light intensity measurement.
U.S. Patent Application Publication 2009/0165285 describes how light intensity in the core of an optical waveguide is measured by providing a light shielding film with a pinhole formed opposite the light exit surface of the waveguide.
U.S. Patent Application Publication 2007/0242921 teaches how to prevent a decrease in near field intensity at the ABS by employing a light scatterer near the ABS to minimize the intensity of back reflected light.